JP2006-73351A discloses a conventional fuel cell system in which it is determined whether a flow rate of an anode gas that leaks out from anode gas flow passages to cathode gas flow passages through an electrolyte membrane (hereinafter, referred to as a “cross leakage amount”) increases or not on the basis of an open circuit voltage difference that was measured when a humidifying amount of a reactant gas to be supplied to a fuel cell is lowered.